Legislation concerning emissions from motor vehicles and heavy goods vehicles makes provision, amongst other things, for a reduction in the amount of nitrogen oxides NOx released into the atmosphere.
In order to achieve this objective, the selective catalytic reduction (SCR) process is known that enables nitrogen oxides to be reduced by injecting a reducing agent, generally ammonia, into the exhaust line. The ammonia may be obtained by thermolytic decomposition of a solution of an ammonia precursor that may be at eutectic concentration. Such an ammonia precursor is generally a solution of urea.
With the SCR process, the high levels of NOx given off by an engine during combustion at optimized efficiency are treated on leaving the engine in a catalytic unit. Such treatment requires the use of a reduction agent at a precise concentration level and of very good quality. The solution is thus measured out accurately and injected into the exhaust gas stream, where it is hydrolyzed prior to converting nitrogen oxide (NOx) into nitrogen (N2) and water (H2O).
To do this, it is necessary to fit vehicles with a tank containing a solution of additive (generally urea), together with a device for measuring out and injecting the desired quantity of additive into the exhaust line.
Given that the aqueous solution of urea that is generally used for this purpose (eutectic at 32.5% by weight of urea) freezes at −11° C., it is necessary to provide a heater device in order to liquefy the solution so as to be able to inject it into the exhaust line when starting under freezing conditions.
In addition, in order to facilitate operation, in order to facilitate restarting the system after freezing, and in order to avoid bursting the often flexible pipes that convey the urea, it is advantageous to purge the pipes at the end of operation of the system. In addition, it is advantageous to purge the injection line (also referred to as the injection channel) connecting the additive tank to the injector.
In the prior art, various purge devices have been provided for this purpose.
For example, an SCR system has been proposed in which the line(s) is/are purged by sucking exhaust gas through the injector with the help of the pump of the system, which for this purpose either operates in the reverse direction or else is associated with a valve enabling flow to be reversed. Thus, that type of known purge device requires either a bidirectional pump (or two pumps each operating in a direction opposite from the other), or else a 4/2 valve (i.e. a valve with 4 ports and 2 positions) controlled electrically by a computer. That type of known device is particularly complex and expensive.